Role of Pre-Existing Normal Faults in Controlling Thrust Ramp Locations: from Example from the Southern Taiwan Orogen
The southern third of the island of Taiwan marks the transition from accretion above the Manila Trench to the south and Eurasia - Philippine Sea Plate collision to the north. With fifteen serial balanced sections through the southern Western Foothills Fold and Thrust Belt (WFFTB), we show that the locations of thrust ramps and subsequent kinematics of the overlying thrust sheets are controlled by large pre-existing normal faults. The normal faults disrupt the ‘normal’ progression of east to west progression in ages of thrust initiation, spawning out-of-sequence thrusts that produce closure earlier than expected in the development of this fold and thrust belt. In addition, incorporation of clipped-off shoulders of inverted normal fault footwall ramps has added significant material (and internal complication) to the overlying thrust sheets. A new regional structure, the sub-Yuching structure is identified in the southern Western Foothills of Taiwan. This structure explains the regional uplift of the Yuching and Tingpiglin synclines, as well as the monocline along the footwall of the Chukou fault. This structure may lie along the northeastern extension of the Manila trench.
The restored position of the pre-existing normal faults places the current trace of the Western Foothills-Slate Belt boundary beneath the Coastal Range, perhaps a frontier prospective region. The restoration of WFFTB's rocks to their depositional locations supports the tectonic model of a crustal-scale thin-skin collisional orogen and rejects a previous hypothesis favoring a deep-rooted Central Range. The thermochronologic data indicate that the depth of involvement of the deformation in the orogen increases to the east but changes character and perhaps deformation mechanism across the Slate Belt -Western Foothills boundary. GPS velocities largely mimic the location and orientations of ramps, both confirming that most folds are actively growing and supporting the location and activity of blind thrusts at the toe of the WFFTB. Mechanical models indicate that even small normal faults are potent stress concentrators if the directly overlying sediments are not relatively weak.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009